Protect Low-Cost Products From ESD

While completely enclosed cases are your best protection, they're not always practical. But there are other techniques that work.

The best protection from electrostatic discharge is a fully shielded enclosure with all I/O and power connectors well bonded to the enclosure. Many low-cost products, however, lack shielded enclosures because of cost. That creates a problem in protecting the circuitry from ESD.

Adding ESD protective devices to the I/O and power traces will help a great deal, but diverting the main current surge (as much as 30A or more), around your circuitry is best. See Figure 1.

So, how do you divert 30A of current around your circuit board without the use of a shielded enclosure? This is the typical situation I've found during many client assignments.

The circuit board is generality mounted in one half of a plastic enclosure with the I/O and power connectors penetrating through the plastic (Figure 2). In this latest assignment, I was asked to determine why simply touching the I/O connectors or plugging in the cables seemed to reset the microprocessor.

Figure 2

This is a very typical situation for low-cost products. The ESD pulse has nowhere to go, but through the circuitry to earth through the power cable.

For most EMC issues, especially ESD, it's important to trace out the path of current. High-frequency currents tend to follow the path of least impedance, and in this case, that path was right through the ground plane to the power cord and back to earth. The current pulse traveling throughout the board coupled into the reset line of the processor and caused it to reboot.

One solution that keeps the cost minimal is to add a metal plane under the circuit board. This plate does not need to be thick metal, but a thin aluminum or steel shim stock will work. It's imperative that the I/O and power connector ground shells be well bonded to this plane, which will form a low-impedance path to earth. Very often, the plane may be attached using the same mounting hardware as the PC board (Figure 3).

Figure 3

Adding a metal plane under the existing circuit board provides an alternative low-impedance path to earth.

One question that may come to mind is: What about portable products without a connection to earth? This is the case for handheld calculators, music players, phones, and many other small, low-cost commercial products. In these cases, the ESD charge tends to bleed off through the capacitance between the plate and earth. For devices such as calculators, where the user presses buttons -- potentially introducing ESD pulses into the top of the PC board -- a gridded ESD shield is used between the buttons.

Any discharges tend to hit the shield, rather than the circuitry. Just be sure the top grid is well bonded to the bottom plate. Adding a low-cost metallic diversion plate is also an easy way to retrofit existing designs to achieve ESD protection.

While a metal plate may be preferred, as most consumer handheld devices have limited internal volume dimensions, the interior of the plastic "shell" could be coated with graphite and bonded to the board main Gnd/plane (remember the dark interior surface of Apple Quadra computer cases?).

One weakness of today's inexpensive consumer handheld devices is their poorly designed AC/DC charger form factor, particularly those with only 2 terminal connector and unshielded wire.

Such "wall warts" don't have an earth ground terminal, which could be tied to the handheld device main ground/plane to give a very low impedance path to earth to discharge ESD events.

In this day and age of "throw away" consumer electronics, the lower the cost of the consumer handheld device, the lower the quality and properties of their plastic case/shell.

Is there an opportunity to develop a standard for portable electronics with requirements for only the use "static dissipative" materials and shielded, static dissipative power cords?

It's doubtful it will happen, particularly now that wireless battery charging technology is starting to enter the market (how about their wall warts and charge surfaces?).